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R. Scott Turner, professor of neurology and director of the Memory Disorder Center at Georgetown University Hospital, points to PET scan results that are part of a study on Allheimer's disease on May 19, 2015, in Washington. (Evan Vucci/AP)

For two decades, scientists have been studying how immunotherapies may help treat Alzheimer's disease. The theory is that antibodies might trigger the brain to destroy a kind of protein that clumps in the brain of patients. The clumps are known as beta-amyloid plaques.

Recently, some large clinical trials of that kind of immunotherapy have failed.An editorial in the New England Journal of Medicine says researchers may be "nearing the end of the amyloid-hypothesis rope."